A thin film transistor (hereinafter, referred to as a “TFT”) having an oxide semiconductor layer of indium gallium zinc oxide (hereinafter, referred to as “IGZO”) or the like as a channel layer has a characteristic in that the TFT has a fast operating speed as compared with a TFT having an amorphous silicon layer as a channel layer and that the TFT requires no crystallization process as compared with a TFT having a polycrystalline silicon layer as a channel layer. Therefore, in recent years, development of TFTs having an oxide semiconductor layer as a channel layer is being actively progressed.
However, the TFT having an oxide semiconductor layer as a channel layer has a problem in that an off-current (the current flowing between a source electrode and a drain electrode in the off state) becomes large. To reduce such an off-current, Patent Document 1 discloses a method for causing a microcrystalline layer to be not easily formed at a backchannel side of the oxide semiconductor layer, by increasing an oxygen concentration at the backchannel side in the TFT having the oxide semiconductor layer as a channel layer. In this way, by causing a microcrystalline layer to be not easily formed at the backchannel side, the backchannel side is made amorphous, and therefore, the off-current of the TFT is reduced.